Anatomy and Physiology Resource Site

14.7 - Blood Pressure and the Kidney

There are three primary methods for the regulation of blood pressure involving the urinary system: The renin-angiotensin mechanism, sympathetic nervous system and antidiuretic hormone (ADH). All three methods have the net result of increasing blood pressure.

Sympathetic nervous system

Activation of the sympathetic nervous system increases blood pressure by:

Increasing cardiac output (increasing heart rate and stroke volume)
Constricting blood vessels in the skin and digestive system (increases peripheral resistance)

Renin-angiotensin mechanism

The juxtaglomerular apparatus

The juxtaglomerular apparatus (JG apparatus) is a small section of tissue in between the distal convoluted tubule (DCT) and the area where the afferent arteriole enters the glomerulus. Lining the side of the DCT closest to the glomerulus are specialized cells called macula densa cells. Granular cells line the afferent arteriole adjacent to the macula densa cells. Both of these cell types play vital roles in the control of blood pressure by the urinary system.

The renin-angiotensin mechanism is triggered by low NaCl in the filtrate, low extracellular fluid volume or low blood pressure.

Granular cells of the JG apparatus sense low blood pressure in the afferent arteriole OR the macula densa cells sense low NaCl in the tubule lumen and trigger the granular cells
Granular cells of the JG apparatus secrete renin into the blood
Angiotensinogen (circulating in the plasma) is converted to angiotensin I by renin
Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE) in the pulmonary circulation
Angiotensin II stimulates aldosterone release from the adrenal cortex
Aldosterone increases Na+ reabsorption in the distal of collecting tubules by increasing passive and active transport of Na+ into the blood.
The movement of sodium into the blood creates an osmotic gradient and water diffuses into the blood, increasing arterial blood pressure

Antidiuretic hormone

Antidiuretic hormone (ADH) secretion is stimulated by a high concentration of solutes in the extracellular fluid (ECF). Unless ADH is present, the distal tubule and collecting duct of the nephron are impermeable to water. ADH allows these two regions of the nephrons to become permeable to water.

When a high ECF concentration is detected ADH is released from the posterior pituitary.
Water channels are inserted into the tubule cells of the distal tubule and collecting duct.
Water diffuses passively from the tubule lumen into the blood.
Blood pressure increases due to increased blood volume.